1. Field of the Invention
The present invention relates to a display device for displaying video signals by thinning depending on the number of horizontal display lines of display means, when displaying interlaced video signals by converting into non-interlaced video signals.
2. Description of the Related Art
Hither to, in a liquid crystal display device of a relatively large size having a display screen of, for example, lateral 640 dots.times.vertical 480 lines, when displaying video signals of NTSC (National Television System Committee) method or PAL (Phase Alternation by Line) method, first the interlaced video signals are converted into non-interlaced video signals, and are then displayed.
FIG. 1 is a diagram showing the composition of a video signal of NTSC method. The video signal of NTSC method, when non-interlaced, comprises 525 horizontal scanning lines in one vertical period, and effective data is available in 483 of these horizontal scanning lines. One horizontal scanning period is 63.56 .mu.sec, and 52.75 .mu.sec thereof is for video signal. The ratio of one screen is lateral:vertical=4:3.
FIG. 2 is a diagram showing the composition of a video signal of PAL method. When non-interlaced, the PAL method video signal comprises 625 horizontal scanning lines in one vertical period, and effective data is available in 575 of these horizontal scanning lines. One horizontal scanning period is 54 .mu.sec, and 52 .mu.sec thereof is for the video signal. The ratio of one screen is lateral:vertical=4:3.
Here is explained a method of converting interlaced video signals into non-interlaced video signals. For example, video signals of one screen by the NTSC interlacing method are composed of 525 horizontal scanning lines as shown in FIG. 1, and are divided into a first field (odd-number field) composed of 262.5 horizontal scanning lines of the odd numbers, and a second field (even-number field) composed of 262.5 horizontal scanning lines of the even numbers, as shown in FIGS. 3(1), 3(2) and 3(3).
As shown in FIG. 4, every 1/60 sec, the first field and second field are displayed alternately, and the scanning lines of second field get into the scanning lines of the first field, and one complete screen (one frame) is composed of two fields. Therefore, 30 screens are displayed in a second. To display the video signals of such interlacing method in a liquid crystal display device, it is necessary to convert into video signals of non-interlacing method. As such method of converting non-interlacing method, 1. the field interpolating method of using field memory, and 2. the line interpolating method of using line memory are known.
FIGS. 5(1), 5(2) and FIG. 6 are diagrams for explaining the non-interlacing converting method by the field interpolating method using field memory. As shown in FIGS. 5(1) and 5(2), first field video signal S1a and second field video signal S2a are respectively written into field memory F1a for first field and field memory F2a for second field. In succession, as shown in FIG. 6, at a double speed of writing, video signals are alternately read out in one horizontal line from the field memories F1a, F2a, and are given to the liquid crystal display device Da to be displayed.
FIGS. 7(1) and 7(2) is a diagram explaining the non-interlacing converting method by line interpolating method using line memory. In the line interpolating method, the video signals of one horizontal line are written into the line memory and are read out at a double speed of writing (double speed conversion), so that two display lines are displayed by video signals of one horizontal line in the liquid crystal display device Da. Therefore, one screen (one frame) is displayed is displayed in the first field (odd-number field), and one screen (one frame) is displayed in the second field (even-number field).
FIGS. 8(1), 8(2) and FIG. 9 are diagrams for practically explaining the non-interlacing converting method by the above double speed conversion. In double speed conversion, as shown in FIGS. 8(1)-(2), video signals of every horizontal line of the first and second fields are read out twice each at a double speed of writing, and are displayed in continuous two display lines. For example, in the first field, data of line m are displayed n line k and line k+1, and in the second field, data of line in are displayed in line k+1 and line k+2. Therefore, as shown in FIG. 9, data of lines m to m+5 are displayed in lines k to k+11, and data of lines n to n+5 are displayed in lines k+1 to k+12.
Here, when displaying the video signals of the NTSC method in a liquid crystal display device having a display screen of lateral 640 dots.times.vertical 480 lines, in both first field and second field, the effective data (video signals) are for 241.5 lines, and the total of 1.5 lines of the beginning and end thereof is overscanned and is set as non-display region, while the video signals of the remaining 240 lines are converted into video signals of the non-interlacing method by the field interpolating method, thereby displaying by converting into video signals of 240.times.2=480 lines.
When displaying the video signals of PAL method in a liquid crystal display device having a display screen of lateral 640 dots.times.vertical 480 lines as mentioned above, the video signals of the PAL method include effective data (video signals) of 575 lines, and all video signals cannot be displayed completely. Therefore, video signals of 575-480=95 lines are thinned in display. In this thinned display, usually, since the central part on the screen is displayed by priority, and the upper and lower parts of the screen are not displayed, and lack of data (video signal) occurs.
To prevent lack of data in a considerably wide range of upper and lower parts, it was proposed to display by thinning at a specific rate by removing one line out of every N line video signals. In such thinned display, however, if the thinning rate N is determined only for the purpose of matching the number of horizontal scanning lines of video signals with the number of horizontal display lines of the display means, without considering the rate of roundness, the image may be distorted vertically or laterally, and the display quality is lowered. Meanwhile, the value of the rate N at which the rate of roundness is 100% is not generally an integer.
FIG. 10 is a diagram for explaining the definition of rate of roundness. The rate of roundness is the ratio of the diameter in the vertical direction to the diameter in the horizontal direction when a circle is displayed on a display device. As shown in FIG. 10, of a circuit displayed in a display device 131 of which pixel pitch is X in the horizontal direction and y in the vertical direction, supposing the number of dots of the diameter in the horizontal direction to be H and the number of dots of the diameter in the vertical direction to be V, the rate of roundness is expressed in Formula 1. ##EQU1##
Here is assumed a liquid crystal display device of a relatively large size of lateral 640 dots.times.vertical 480 lines composed corresponding to the video signals of the NTSC system, for example. According to the composition diagram of video signal of NTSC method in FIG. 1, the sampling period necessary for setting the rate of roundness to 100% must be, when the pixel pitches are equal in the horizontal direction and vertical direction, must be as follows. ##EQU2##
Generally, in order to obtain a sampling clock in a liquid crystal display device, using a PLL (phase locked loop) circuit, a clock of an integer multiple of one horizontal scanning period is created. In this case, the doubling ratio is as follows. ##EQU3## Therefore, the rate of roundness when the video signal of NTSC method is displayed in this liquid crystal display device is ##EQU4##
When video signal of PAL method enters the same liquid crystal display device, the period of the produced sampling clock is, according to one horizontal scanning period, as follows. ##EQU5## Hence, the data for the portion of one horizontal display line is ##EQU6## Therefore, to realize the display of 100% roundness, the 575 effective horizontal scanning lines must be compressed to 473 lines. ##EQU7##
When thinning the horizontal display lines at a rate of one line in every N lines according to the conventional method, the compression rate is 20% when N=5, or 16.67% when N=6.
The target compression rate is ##EQU8## and, accordingly, if thinned at a rate of one out of every five lines, the compression rate is too large, and the image is extended laterally, or when thinned at a rate of one out of every six lines, the compression rate is too small, and the image is extended vertically. Therefore, the thinning rate for achieving 100% roundness is EQU 5&lt;N&lt;6 (9)